1. Field of the Invention
This invention concerns a radar-like device for use in production wells, that is arranged for detecting the oil/water contact in a reservoir rock.
More specifically the invention comprises: (a) a transmitter antenna for electromagnetic waves that is placed fixedly by a production tubing inside a geological formation; and (b) receiver antennas, also placed near a production tubing. This radar-like device is capable of detecting reflectors which constitute by electrically conductive surfaces inside the reservoir. One such surface of particular importance is the oil/water contact. The water front, in most instances, constitutes a relatively sharp transition between oil-bearing sand with high resistivity, to water-filled sand with low resistivity, thereby consisting a reflector.
2. Description of the Related Art
Borehole logging tools utilizing the radar principle are known from U.S. Pat. No. 4,670,717, U.S. Pat. No. 4,814,768, U.S. Pat. No. 4,297,699, U.S. Pat. No. 4,430,653, and GB 2,030,414. Some of these patents use methods where one must estimate a wave propagation speed in order to be able to interpret the radar signals.
Schlumbergers U.S. Pat. No. 5,530,359 xe2x80x9cBorehole logging tools and methods using reflected electromagnetic signalsxe2x80x9d describes a logging tool with pulsed radar signals being transmitted from a transmitter antenna in a separate vertical section. The logging tool freely hangs in the borehole from a cable or in a coiled tubing. Linear antenna elements are arranged parallel to the tool""s long z axis. Electromagnetic pulses are emitted with a center frequency of 40 MHz and with a highest frequency of 120 MHz. This pulse is radiated in all directions in the formation and is reflected by structures in the formation back to the tool in the borehole. The transit time for the pulse out to the structure and back to the tool is used for determining the distance between the reflecting structure and the borehole. Directional information is achieved by arranging receiver antennas about the entire periphery of the tool so that the reflecting structure""s direction may be found by taking differences between the reflected signals. These differences may be calculated by means of electronic circuits, or subtraction may be performed by directly differentially coupled receiver antennas. One method for calculating the reflected signals"" directions is given. One disadvantage with Schlumberger""s patent U.S. Pat. No. 5,530,359 is that the instrument applies pulsed electromagnetic waves. this entails spreading of frequency components already in the emitted signal, and thereby the emitted signal pulse has a continuously varying group velocity. The reflected signal becomes smeared out causing an unclear image of the reflecting structures. Close reflecting structures will also dominate over the more remotely reflecting structures remote structures may be difficult to detect if the closer rocks have a relatively high conductivity/low resistivity. Another disadvantage of the Schlumberger instrument is that as it is not fixedly arranged by the geological formation, it is impossible to trace changes of the electrical parameters in the formation over a period of time, e.g. from one data to another. The instrument is also not capable of being applied in production wells or in injection wells.
Another apparatus is described in U.S. Pat. No. 5,552,786 xe2x80x9cMethod and apparatus for logging underground formations using radarxe2x80x9d, (Schlumberger). U.S. Pat. No. 3,552,786 describes a logging tool which partially solves the problem with the electromagnetic wave speed in the formations which are to be logged. The apparatus transmits an electromagnetic pulse insert a comma in close contact a borehole wall, into the formations and receives the direct wave in a predetermined distance along the borehole string from the transmitter. Thus the wave speed for the direct wave through the rocks (which may be invaded by borehole mud) can be calculated and the reflectors distances from the transmitter/receiver system may be calculated more exactly than if one had only an estimate of the wave speed.
U.S. Pat. No. 4,504,833 xe2x80x9cSynthetic pulse radar system and methodxe2x80x9d describes a synthetically pulsed radar generating a plurality of signals of different frequencies simultaneously. The response from the formation those different frequencies simulates parts of the Fourier spectrum which would have been measured if one emitted a very short pulse which, according to the mathematical background, should be very broad in frequency spectrum. The system can however be used on board a vehicle because it is able to generate all the component signals simultaneously.
U.S. Pat. No. 4,275,787 xe2x80x9cMethod for monitoring subsurface combustion and gasification processes in coal seamsxe2x80x9d describes a radar for detecting a combustion front in a geological formation, for example a coal bearing formation. Because resistivity generally increases with temperature, such a combustion front will display high resistivity and constitute a very large contrast with respect to the coal bearing formation which normally will display low resistivity. The attenuation: (a) exceeds 100 dB/wave length in the combustion front; (b) is one dB/wave length in xe2x80x9cPittsburgh coalxe2x80x9d; and (c) is 3 dB/wave length in xe2x80x9cBritish coal.xe2x80x9d A detection range of the combustion front is 100 meters, an unrealistically large distance when one takes in consideration the conditions in an oil well with the attenuation of the signal being much higher and where it is very difficult to detect reflecting surfaces only one to two out in the reservoir. A swept signal varying continuously between a lowest and a highest frequency is emitted. Whereas the combustion front is displaced, one will by subtraction of the received signals be able to see a change corresponding to the difference between the signals. However, that patent does not consider the need for tuning the transmitter antennas when the transmitter antennas are situated very close (e.g. a few millimeters) to a metallic tubing surface (e.g., the linear tubing or a completion tubing.
The invention is made partially on the background of the potential problems which could arise in connection with petroleum production on the Troll oilfield in the North Sea. As described below the resistivities in the actual geological formations are relatively lower with respect to the conditions described in the known art. Thus, it is not feasible to perform detection by means of electromagnetic waves according to the known art.
1. Expected Resistivity
A map of the Troll Oilfield generally covering the licence blocks 31/2, 31/3, 31/5 and 31/6 is shown in FIG. 3a. Resistivity data are available from five wells: 31/2-2 (FIG. 3b), 31/2-4 (FIG. 3c), 31/2-5 (FIG. 3d), 31/2-6 (FIG. 3e), and 31/2-7 (FIG. 3f). The graphs display resistivity in xcexa9m as a function of logging depth in generally vertical wells through the reservoir rocks. The oil/water contact, hereafter called xe2x80x9cOWC,xe2x80x9d is defined in the wells by the depths marked in the respective graphs. The distribution of resistivity with respect to depth is markedly different from well to well. In 31/2-2 the resistivity Rt varies between about 3 xcexa9m and 1.3 xcexa9m over the OWC while Rt in well 31/2-4 decreases from 100 xcexa9m to 1 xcexa9m over the OWC. In well 31/2-5 the resistivity varies between 40 xcexa9m and 80 xcexa9m before it starts to decrease monotonously, about 1 meter above the OWC. By the OWC the resistivity falls by about 7 xcexa9m. The development in well 31/2-6 is characterized by a relatively strong xe2x80x9cripplexe2x80x9d between 8 xcexa9m and 14 xcexa9m, even though the resistivity drop is clear by the OWC. Well 31/2-7 has a low and relatively little varying Rt in the area between 7 meters above the OWC and down to the OWC, with a maximum of approximately 2 xcexa9m and falling to approximately 0.4 xcexa9m just before the OWC.
The resistivity curves show that local variations in Rt may be much larger than the drop in Rt that takes place at the OWC. Because the conductivity of the formations generally arises from saline water in pore spaces or conductive schists, local variations may be due to varying reservoir quality in the form of a combination of clay mineral content and porosity. Parameters like local lithology, texture, facies and overpressure will also affect the resistivity. Resistivity tools are generally quite precise and give repeatable measurements. Generally the depth resolution is small, about 10 cm per measurement point, and the logs are smoothed to a certain degree by the contact assembly of the instrument (so the local formation resistivity will vary more than shown by the logs).
2. Expected Dielectric
No dielectrical logs are available from the Troll Oilfield. Here, dielectrical data are applied based on estimates of the known dielectrical properties in sandstone, oil and water. We select a dielectric constant for rock, xcex5rock=7. When xcex4 is 0.20 (20% porosity), the dielectric constant for oil saturated sandstone xcex5ro equals 5.82. Accordingly, xcex5ro=6 is a reasonable estimate of the dielectric constant for oil saturated sandstone.
The relative dielectric constant for sea water, using frequencies which apply to this invention, is xcex5water =80 (King and Smith, 1981). The dielectrical constant in water saturated sandstone is xcex5rw=13. FIGS. 4a, b, c, d, and e display estimated distributions of relative dielectrical values based on the water saturation in a five meter transition zone generally over but slightly across the OWC in the same wells represented in FIGS. 3b-f. The scale indicates that the relative dielectric constant is from about 6 to about 13.
3. Wave Propagation in a Conductive Transition Zone
FIG. 5a displays an attenuation graph for electromagnetic waves in the frequency range between 1 MHz and 200 MHz. xcex5r=6 whereas the resistivity RDC is varied in steps of 5 xcexa9m from 5 xcexa9m to 30 xcexa9m. The higher the resistivity, the more xe2x80x9ctransparentxe2x80x9d the rocks become to electromagnetic radiation.
FIG. 5b, with the same frequency range, displays graphs with a constant RDC=30 xcexa9m whereas xcex5r varies from 6, 8, 10, 12, 14, to 16. One can see that the dielectric constant has less effect on the attenuation than the resistivity. The graphs show that the attenuation is more than 10 dB/m for 30 xcexa9m and frequencies above 12 MHz. An attenuation above 10 dB/m yields a return attenuation of more than 100 dB from a reflector at a distance of 5 meters.
FIG. 5c displays a section of the frequency range from FIG. 5a, between 1 and 16 MHz. The attenuation is still high for resistivities below 10 xcexa9m even in this low frequency range. FIG. 5d displays the waves"" phase velocities as a function of frequency between 1 and 16 MHz. Thus, on the background of the attenuation, the frequencies applied in a preferred embodiment of the invention may be between 1 and 16 MHz. Within this frequency range, the phase velocity varies strongly with the resistivity which may give strong dispersion of an electromagnetic signal with a broad frequency content.
4. Reflection and Backscattering
All horizons with electromagnetic resistivity contrast in the well will result in reflections. Particles with higher conductivity (e.g., metal oxides), will entail a dispersion of the electromagnetic waves. Near horizons will become detected more strongly than remote horizons if the resistivity contrasts are equal (due to approximately spherically geometric spreading). This means that the reflexes from the resistivity contrast of the OWC may be masked behind a large number of strong reflexes from local resistivity contrasts in the sandstone in the oil zone over the OWC. For example, the resistivity contrasts represented by the gradients (i.e., derivatives) of Rt at 1578 meters and at 1580.5 meters depth in FIG. 3e will give strong reflexes which initially are not different from the reflex at the OWC.
One purpose of this invention is to provide a system to measure the depth of the oil/water or the gas/water contact in a petroleum reservoir by means of electromagnetic waves.
Another purpose is to provide an instrument arranged for registering and mapping the distribution of resistivity in the petroleum reservoir around the well and, to apply this resistivity to geological interpretations of the reservoir.
The above mentioned problems are remedied by means of the present invention being a device for radar detection in a well in a geological formation. The new and inventive invention includes:
(a) at least one transmitter antenna for emission of electromagnetic waves arranged along a tubing string in a fixed and permanent position, and shielded with respect to the geological formation, and
(b) at least one (and preferably more) receiver antenna for receiving the reflected electromagnetic waves arranged along the tubing string in a fixed and permanent position, and unshielded with respect to the geological formation.
Additional features of the invention will become apparent to one of ordinary skill in the art upon reading this disclosure.